Microwave ovens have become a well-known household cooking appliance. Such appliances represent a variety of advantages to the user, not the least of which is the relative speed with which cooking can be accomplished. While such speed in on the one hand an advantage, it can be at the same time a disadvantage in that a fairly small error in cooking time can result in an unacceptably underdone or overdone food product.
Heretofore cooking in a microwave oven has been primarily controlled through the control of time. Microwave ovens include some sort of timer wherein a given time in minutes and seconds is chosen as appropriate for the food type and quantity to be cooked. Such times can be selected on the basis of cookbooks, or on the basis of experience based upon trial and error.
However, the usual object in cooking food is to bring the food to some state of doneness. The doneness of food is not generally measured in terms of time of cooking, but is far more commonly measured in terms of the internal temperature of the food. Therefore, microwave ovens which are controlled solely on the basis of time require considerable instruction to the user in order to allow the user to make the necessary conversion from temperature of the food to the time required to reach that temperature.
It can therefore be understood that it would be highly advantageous to provide a microwave oven in which the oven can be controlled directly through the temperature of the food being cooked without regard to the elapsed time factor. Various schemes have been suggested in the prior art in an effort to develop such an oven, but because of inherent disadvantages in such prior art suggestions ovens incorporating them have not found wide commercial acceptance.
One such class of techniques involves the measurement of various parameters of the air surrounding the food. For example, it has been suggested to measure the humidity of the air in the cooking cavity, or of the gases exhausted therefrom, on the theory that the water vapor given off by the cooking food indicates a relative measurement of temperature. Other schemes have suggested measuring the temperature of the exhaust gases as an indication of the temperature of the food itself. The clear disadvantage of these techniques is the fact that the control parameter is something other than the temperature of the food itself, although it may be somewhat related to the food temperature. However, since the degree of doneness is to be determined by the temperature of the food itself, techniques which do not involve the direct measurement of that temperature suffer from inherent inaccuracies.
Another class of techniques that have been suggested is the use of sonic waves and the acoustics of the cooking chamber to sense the presence of moisture in the air. Clearly this type of technique incorporates the same kinds of disadvantages found in the humidity measurement techniques discussed previously.
It has also been suggested to control the cooking by placing a small amount of some type of material in addition to the food into the cooking cavity, the additional material being directly connected to the control circuit. Again, the approach is to measure a parameter other than the temperature of the food itself and to attempt to extrapolate from that quantity to the food temperature.
It also has been suggested to provide a thermometer for insertion into the food product, which thermometer may be constructed of nonconductive materials with nonconductive expansible fluid therein. However, such a thermometer is capable only of indicating the temperature of the food product, but is not capable of exercising any direct control over the power supplied to the cavity. Therefore, the user must pay constant attention to the thermometer and manually terminate the application of power upon reaching the desired temperature.
It is therefore an object of the present invention to overcome the shortcomings of the prior art techniques and to provide a means for controlling the power supplied to a microwave oven directly in response to the temperature of the food being cooked in the oven.
A further object of the present invention is to provide a temperature responsive means of controlling a microwave oven whereby the power supplied to the oven may be automatically terminated upon the reaching of a preselected temperature of the food product.
Yet another object of the present invention is to provide a temperature responsive probe insertable into a foodstuff to be cooked in a microwave oven, which probe is adapted to provide a signal to interrupt the application of microwave power upon reaching a predetermined temperature.
Yet another object of the present invention is to provide a temperature responsive food probe useable in a microwave environment and which is capable of transmitting a signal in response to a change of state of the probe conditioned upon its temperature.